A distributed process application server system, such as the commercially available WinFrame.TM. system manufactured by Citrix Systems, Inc. of Coral Springs, Fla., and shown in FIG. 1, includes a master server 100 and a number of Win32 application servers 200. Each application server 200 may support multi-tasking of several applications at a remotely located workstation. Master server 100 controls access to the host systems resources that are to be shared between the Win32 application servers and provides the application servers with timely processed application data so that the user at a remote workstation is given the illusion that the user's application program is running locally at the workstation. This requires that all communications between master server 100 and all of the associated application servers 200 be efficiently carried out so that the effective throughput of the WinFrame.TM. system is maximized.
The WinFrame.TM. system's master server process handles the creation, deletion, connection and disconnection of remote user workstations. Separate application service processes are created, destroyed, and have their internal connection state manipulated by master server process 100. Command communications between master server process 100 and each Win32 application server use synchronous procedure calling techniques.
Synchronous procedure calling requires that when a master process places a call to a server process, the master process awaits a reply from the called server process before continuing on to its next task. Therefore, if an application server process 200 does not respond to a call from the master server process 100 (e.g. due to a temporary hardware failure or driver software error), master server process 100 is left in a virtual deadlock state. Because multiple Win32 application server 200 processes are dependent on master server process 100, the effects of a deadlock between the master server process and any application server process become widely spread throughout the system.
The present invention provides an effective means for avoiding deadlock while maintaining the desirable features of the synchronous calling procedure.